Tape libraries have historically been the primary storage devices for amassed digital data. This has been due in part because of the tape libraries' ability to store considerably large amounts of data in a cost-efficient and data-stable manner. Other types of storage systems containing alternative media such as disc drives, however, are beginning to gain momentum as a consequence of advancements in the technology. Disc drive-based systems generally facilitate an additional level of flexibility in the way data is stored. Depending on the configuration of the storage system, the random access functionality of disc drives can dramatically speed up data transfer between a host and storage system. Such advances are evident in a Virtual Tape Library (VTL) which is a leading alternative to the traditional tape library.
VTLs are generally promoted as faster, more versatile backup systems than the traditional tape-based libraries. A VTL works by emulating legacy tape libraries by operating with standard tape backup software, such as Veritas backup software from Veritas Corporation of Mountain View, Calif. Put simply, a VTL presents the appearance of a tape library of almost any make and model to a host.
FIG. 1 shows a prior art block diagram of the relationship between a VTL 100 and a host computer 102 over a network communications path 106. The VTL 100 is comprised of an array of disc drives 108 often configured in a RAID (Redundant Array of Independent Discs [drives]). Here, for purposes of simplicity, the VTL RAID system 108 is comprised of four disc drives 104. The disc drives 104 function as one virtual disc drive 120 capable of sharing a single article of data, such as a file, across multiple disc drives 104. This feature enables data to be saved and retrieved at fast rates because an article/s of data can be accessed in a parallel fashion. In the event a disc drive 104 fails in the RAID 108, the data on any given drive 104 can be reconstructed from redundant data on other disc drives 104 which provides a bonus feature over a conventional tape library. As previously discussed, the VTL 100 appears to be a tape library 110 to the host computer 102, which in this configuration, is constructed with three tape drives A 112, B 114 and C 116 and twenty-four tapes, or tape articles, such as tape article [1] 122.
In one example, the host computer 102 can communicate over the communications path 106 with the VTL 100, to query the configuration of the VTL 100. The VTL 100 can respond indicating that it is a tape library 110 with twenty-four tape articles and three drives (A 112, B 114 and C 116). The host 102 can communicate a request to send data packages to tape articles [1] 122, [2] 124 and [3] 126 to be loaded in tape drives A 112, B 114 and C 116, respectively. The host 102 receiving such a positive response over the communications path 106 from the VTL 100 sends data via the communications path 106 to the VTL 100 to be saved on tape articles [1] 122, [2] 124 and [3] 126. The VTL 100, in turn saves the data across the disc drives 104 in the RAID 108 as if they were three tape articles [1] 122, [2] 124 and [3] 126 on the virtual drive 120.
Unlike conventional tape libraries wherein tape articles can be removed from the library and archived and replaced with new or blank tapes to provide additional storage space, the VTL 100 is typically limited to the size of the RAID 108. This has been addressed by enlarging the RAID with additional disc drives 104 or adding additional RAID systems. The components and effort required to enlarge the RAID, however, often is substantially greater than that associated with increasing storage space in conventional tape libraries.